This application relies for priority upon Korean Patent Application No. 2001-78667, filed on Dec. 12, 2001, the contents of which are herein incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to the fields of fiber optics, and more particularly, to a method for forming two thin conductive films that can be used as electrodes for poling on a single mode fiber or a multimode fiber.
2. Description of the Related Art
A crystal material such as LiNbO3or BaTiO3 has been widely used as the optical nonlinear material. An amorphous material has no inversion symmetry, which means vanishing electro-optic and second order nonlinear coefficients. However, since amorphous silica glass materials were reported to have electrooptic and nonlinear optical properties by poling them under several different conditions, a lot of researches and progresses have been focused on the optoelectronic devices made of glass materials, for a possibility of integrating all glass, or specifically all fiber, active devices into a system. It has been suggested that packaging cost and optical power loss can be significantly reduced by making all the active devices out of glasses or fibers and integrating them into a system. Modulation speed and transmission rate can also be enhanced by integrating the active devices made of glasses or fibers into a system.
To induce and enhance the second order effect on a glass material, two techniques are well-known, thermal poling and UV poling. In thermal poling, a high electric field of a typical value of xcx9c100 V/xcexcm is applied to a glass material with heating to a desired temperature, typically 250xc2x0 C. to 300xc2x0 C., then the material is allowed to cool to room temperature with the high electric field still applied. In UV poling, a glass material is irradiated by UV light, typical wavelength of 150 nm to 400 nm, with the high electric field applied. Both methods, thermal and UV poling, may be applied together to a glass material at the same time.
To apply a high electric field to a single mode fiber, a typical diameter of 125 xcexcm, requires well-tailored techniques and clever designing of electrodes to be located as close as allowed to the core of a fiber, and also requires isolation schemes to allow xcx9c100 V/xcexcm high electric field without breakdown.
The prior arts introduce several different designs to locate electrodes in appropriate positions with desired patterns [U.S. Pat. No. 5,617,499]. With reference to FIGS. 1A through 1D, a first electrode 12 is formed on a wafer 11. A polyimide 13 is applied in order to attach a D-shape fiber 10 to the wafer 11 with the flat side down. Then, as a dielectric insulator, a thick polyimide layer 14 is deposited. The upper parts of the polyimide layer 14 and the fiber 10 are polished and a polished side is formed on the opposite side of the flat side of the fiber 10. A second electrode 15 is formed on the polished side. If the electric field is applied to the fiber 10 through the first electrode 12 and the second electrode 15, the electro-optical coefficient is induced.
Though adequate in some respects, these kinds of prior arts, which consist of the processes of the permanent attachment of the fiber to a substrate, polishing, depositing a polyimide layer as a dielectric insulator, and spin coating, have inevitable shortcomings. The permanent attachment of the fiber 10 to a substrate 11 requires additional packaging and handling costs for later use of a device and also reduces compactness. Depositing electrodes 12 and 15 not on the fiber surface itself, but on the polished surface and the substrate may result in an unstable and non-uniform electric field, which is caused mainly by the imperfectness of the polished surface and the polyimide glue layer 13 between the fiber 10 and the substrate 11, which may also be a cause of a lack of reproducibility. Polishing a fiber is a difficult and costly process because polishing accuracy must be high due to the small size of the fiber. This is especially difficult and costly for long fiber distances. Depositing a polyimide layer 14 as a dielectric insulator also has a shortcoming that the polyimide layer 14 allows some amount of leakage current, especially in a high temperature under a high voltage. In our experiment we suffered a serious problem where extended ends 18 and 19 of the fiber 10 away from the substrate 11 were really weak such that the spin-coating was hardly allowable. Here, the extended ends of the fiber are necessary pieces that are used for splicing with external fiber ends.
FIG. 2 shows another prior art to induce and enhance electro-optic and nonlinear effects on a fiber 30 [U.S. Pat. No. 5,966,233]. The fiber 30 has two holes running parallel to a core 33 with pre-designed distances, where the two holes accommodate two thin electrode wires 31 and 32 inserted from each end of the fiber. Through the electrode wires 31 and 32, the electric field is applied to the fiber 30 for UV or thermal poling, or both together. According to the method, the electro-optical coefficient was obtained to the value of maximum 6 pm/V that is practicable enough to facilitate the configuration of a nonlinear element using a silica fiber. Coupling the fiber to other fibers is difficult because the electrodes 31 and 32 must exit the fiber, preventing direct butt coupling or fusion splicing to other fibers. It is also very difficult to push the fragile electrode wires into the end of the fiber, henceforth the manufacturing cost and time are significant drawbacks. Inserting electrode wires from each end of the fiber means that the modulation frequency, if embodied into a modulator, is limited to low values since a high-speed traveling wave geometry is not possible.
Prior art U.S. Pat. No. 5,966,233 disclosing the design shown in FIG. 3 suggests some improvements to the art in FIG. 2, removing the problem of inserting electrode wires into the holes, thereby making possible fusion splicing to other fibers and allowing a high-speed traveling wave geometry. In this method, grooves 64 and 65 are formed on the surface of a fiber 60 along the length direction of the fiber. Electrodes 69 are placed on the grooves 64 and 65. The electric field is applied through the electrodes for poling.
Both the prior arts shown in FIGS. 2 and 3, however, still have a serious drawback that a patterned electrode such as a periodic pattern to meet the quasi-phase matching condition is very difficult to realize. Up to the present time, as far as we know, there have been no publications reporting that a significant result of nonlinear effects, such as Second Harmonic Generation (SHG), Difference Frequency Generation (DFG) or Sum Frequency Generation (SFG), four wave mixing etc., was obtained using a patterned electrode under the schemes shown in FIG. 2 and FIG. 3. One shortcoming related to the prior arts shown in FIG. 2 and FIG. 3 is that the diameters of their fibers, xcx9c300 xcexcm, to accommodate two electrode wires are much larger than a standard single mode fiber, xcx9c125 xcexcm, meaning that, although it is not impossible, the fusion splicing of their fibers with the standard single mode fiber is very difficult and allows a lot of power loss.
To solve the problems described above, it is an object of the present invention to provide a method for forming two thin conductive films that can be used as electrodes for poling on an optical fiber, addressing the problems of prior arts.
To achieve the above object, in accordance with the present invention, two conductive films isolated electrically from each other are deposited on the surface of a fiber that may be used as an optoelectronic device in an optical communication system. According to the present invention, D-shape fibers are attached into grooves on a silicon wafer substrate, and photoresist is used as glue. The wafer is prepared by forming grooves by wet-etching and coating photo-resist in the grooves to accommodate the fiber without a step difference to the surface of the substrate. After conducting a well-designed cleaning procedure to remove the dirty extra photo-resist on the top surface of the fiber during the attachment procedure, a photoresist pattern is formed on the top surface of the fiber by a photolithography process, where specific conditions must be established in each step.
After wet-etching some amount of the fiber on the patterned area, which is needed for lifting off a metal film deposited on the unnecessary area, a metal film is deposited over whole area of the wafer. Removing photo-resist by a heated stripper solution leaves a metal film only on the patterned area of the fiber and detaches the fibers from the grooves of the wafer. The second metal film on the other side of the fiber can be formed by the same procedures as the first except that the deposited surface of the fiber must be attached to the grooves upside down, where the size of the grooves for the plat side down is different from the size for the round side down. In the final step, an insulation film such as an oxide layer is deposited on the metal films to protect them from electrical and mechanical damages. The method of the present invention can be applied to a circular single mode fiber, a multimode fiber or a D-shape fiber.
A fiber manufactured according to the above processes is almost completely free from the drawbacks mentioned above, free from the permanent attachment to a substrate, free from the polishing process, and free from the polyimide insulating layer, and such a fiber allows fusion splicing and direct butt coupling to the standard single mode fiber, allows the high speed traveling wave geometry, and allows patterned films of any shape. Although the two metal films (further, conductive films) isolated electrically are used as electrodes to apply an electric field to an optical fiber for poling now, they are expected to be used in various applications in the future. For example, the conductive films may be used to apply currents to induce the difference of temperatures between two films. In addition, if the two conductive films are made up of different materials, then the difference of reflection, refractive index and stress across the fiber can be induced.